您可以使用WP Query 具有meta_query 参数如下。

$args = array (
    \'post_type\'              => array( \'post\' ),  // YOUR POST TYPE
    \'meta_query\'             => array(
        array(
            \'key\'       => \'country\',  
            \'value\'     => $your_country,  // THE COUNTRY TO SEARCH
            \'compare\'   => \'LIKE\',  // TO SEARCH THIS COUNTRY IN YOUR COMMA SEPERATED STRING
            \'type\'      => \'CHAR\',
        ),
    ),
);

// The Query
$query = new WP_Query( $args );

Edit:

$countries_to_search = \'usa,africa,poland\'; // YOUR COMMA SEPARATED STRING
$countries = explode( \',\', $countries_to_search ); // CONVERT INTO ARRAY OF COUNTRIES
$meta_array = array();
$meta_array[\'relation\'] = \'AND\';  // DECLARE RELATION --> YOU CAN USE \'OR\' AS WELL  

foreach($countries as $country){  // CREATE QUERY FOR EACH COUNTRY IN ARRAY

    $meta_array[] =  array(
        \'key\'       => \'country\',  
        \'value\'     => $country,  // THE COUNTRY TO SEARCH
        \'compare\'   => \'LIKE\',  // TO SEARCH THIS COUNTRY IN YOUR COMMA SEPERATED STRING
        \'type\'      => \'CHAR\',
    );

}

$args = array (
        \'post_type\'     => array( \'post\' ),  // YOUR POST TYPE
        \'meta_query\'    => $meta_array,  // ARRAY CONTAINING META QUERY
        );

// The Query
$query = new WP_Query( $args );